1. Field of the Invention
The present invention relates to a heat treatment apparatus having a wafer boat for heat-treating a semiconductor wafer, and more particularly, to an improvement of a boat for heat treatment adapted to be loaded in a vertical furnace.
2. Description of the Related Art
Since a vertical CVD apparatus is designed so that a wafer boat can be taken into or out from a process tube without touching the inner wall of the tube, it advantage, the apparatus of this type is conventionally used for the manufacture of a large scale integration semiconductor device, such as a VLSI.
In order to form a thin film with a uniform thickness on a large number of semiconductor wafers at a time, in a vertical furnace, a uniform flow of process gas must be produced with a minimum disturbance in the process tube.
In the vertical CVD furnace, in contrast with this, batch processing is executed, so that as many wafers as possible are expected to be processed at one time. Therefore, various components (supporting members and reinforcing members) of the wafer boat arrest a uniform gas flow in the process tube. Conventionally, there has been an attempt to improve the uniformity of wafer-surface film thickness during the film forming operation by variously changing the shape of the wafer boat.
Disclosed in Published Unexamined Japanese Patent Applications Nos. 58-108735 and 61-201695 is a wafer boat having a wafer loading disk whose diameter is larger than the wafer diameter. With use of this wafer boat, the deviation of wafer-surface film thickness can be decreased when phosphine (PH.sub.3) is added while a polysilicon film is being formed, and the film is finally doped with phosphorus to form a phosphorus-doped polysilicon film.
In a VLSI having a memory capacity of 1 to 4 megabits, however, the circuit pattern is so fine that it requires a high uniformity of wafer-surface film thickness. The aforesaid wafer boat cannot, however, fulfill this requirement. The larger the wafer diameter, in particular, the less uniform the film thickness at the peripheral edge portion of the wafer is liable to be. Modern wafers tend to switch from 6-inch versions to 8-inch versions, requiring effective measures to ensure the uniformity of wafer-surface film thickness.